There’s been quite a few RESTCONF bug fixes since ConfD 6.4 (10+). Nothing in particular sticks out but the list include only externally reported tickets and it’s certainly possible, likely even, that engineering have made other improvements as well.
I haven’t tried with ConfD 6.4 myself, If you need more information or a patch on ConfD 6.4, it’s best if you file a ticket with ConfD support.
I tried with ConfD 6.4, with fresh install on Ubuntu 16.04 and it works fine with the same curl command you used.
I’m not sure what happens in your case. It could be due to your configuration data or other YANG models you have installed. In any case, as far as I can tell you’re not doing anything wrong and it’s probably best if you contact support.
Could you attach the curl version and output of your test session (with curl -v option) to the thread, pls? Not sure sure if there is a curl version mis-match or some curl option that could be causing this.
Saw this on 2 separate CONFD installations for different nodes in the tree.